Acta Geodaetica et Geophysica最新文献

Liquid nitrogen (LN₂) ultra-low temperature fracturing presents a broad application range for geothermal energy development. Therefore, studying the effect of LN₂ cooling on the pore characteristics of high-temperature limestone is critical. In this study, Rizhao limestone obtained from Shandong Province was selected as the test object and subjected to heat treatment at different temperatures (25–600 °C) to investigate the changes in the pore characteristics of limestone after rapid LN₂ cooling based on nuclear magnetic resonance. The total porosity of limestone continuously increases due to the continuous expansion of original fractures in the limestone and the generation of new fractures caused by thermal stress resulting from high temperatures and the rapid LN₂ cooling. For heat treatment of 600 °C, no high amplitude is observed in the damage process. The fractal dimension of permeable pores (D_P) initially decreases and then increases with an increase in the quenching temperature difference, and the fractal dimension of total pores (D_T) shows a general decreasing trend. The compressive strength increases with an increase in D_T. Acoustic emission (AE) technology was used to monitor the deformation and failure of rocks. The low amplitude of AE is mainly distributed in the low-frequency band.

We show in this note a simple exercise to overcome a common preconception among Geodesy students about the “best” reference ellipsoid. This helps students to overcome their previous ideas based on a purely geometric vision of the reference ellipsoids and advance to a more physical point of view, where the Earth’s gravity plays a fundamental role.

We analyze theoretically ultra-low frequency electromagnetic noise caused by deformations of seabed and porous coastal rocks subjected to incident long oceanic waves. A variable pressure on the seabed due to propagation of long gravity waves (LGWs) gives rise to variations in pore pressure gradient followed by groundwater filtration in pores and channels of porous rocks. These processes result in the generation of telluric electric currents in water-saturated porous rock of the seashore due to electrokinetic effect. In the model a displacement of the sea surface in LGWs is described in the "shallow water" approximation. A set of basic equations describing rock strain and electrokinetic effect is solved in quasi-static approximation. The telluric electric field in the porous rocks of coastal zone are found as a function of depth and distance to the coastline at different frequencies of LGWs. The theoretical analysis has shown that telluric electric noise produced by the LGW can exceed the level of natural electric noise during geomagnetically quiet period in a coastal strip about several tens of meters.

The present paper introduces a Legendre polynomials based interval inversion method for processing multi-borehole logging data. The method allows the determination of lateral changes of the layer-thicknesses together with the vertical and lateral variations of petrophysical parameters along a 2D cross-section of several boreholes. The method is assessed using noisy synthetic measurements of a petrophysical model made of two-layers structure related to hydrocarbon bearing formations. The numerical experiments aided to investigate the stability and convergence of the 2D interval inversion procedure. To ensure the accuracy and reliability of the inversion results, the misfit of data and model distance are tested, beside the calculation of estimation errors and correlation coefficients. A large amount of input data relative to the number of unknowns results in a high overdetermined ratio, consequently more precise estimates are obtained in stable and convergent procedure than in conventional local (1D) inversion schemes. The feasibility of the 2D interval inversion method is shown by analysing in-situ well logging data acquired in four wells situated in an Egyptian hydrocarbon field.

Works on establishment of the spirit levelling network in the Balkan countries were ruled by the political and economic circumstances. This was subject to the analysis since the Austro-Hungarian Monarchy period, over former Yugoslavia in all of its political forms, down to the present countries of the Balkan Peninsula. Heights differences and heights are part of the II Levelling Network of High Accuracy (II.NVT network). The works on establishment of II.NVT network have been organized and performed by the Federal Geodetic Administration of former Yugoslavia and the Geodetic Administrations of the accompanying Republics. The Military-Geographic Institute from Belgrade had performed gravity measurements over one part of the levelling network. “The II.NVT network data are the subject of computer processing, modern analyses, fundamental and applied scientific researches and application in solving some practical tasks” (Rožić in Acta Geodaetica et Geophysica Hungarica 36(2):231–243, 2001). Calculations of accuracy of corrections in different height systems will be presented in this paper. The subject of this analysis is the error sources in the corrections for the orthometric, normal, and normal orthometric heights. The error sources that accompany the spirit levelling procedure are, for the most part, examined with the assumption that the height difference may be determined with the accuracy of 1.0 mm/km. The corrections in different height systems will be made through the function of a link to the directly/indirectly measured values. The accuracy of the correction will be presented by applying the law of error propagation on the data collected in the II.NVT network for the territory with the characteristic relief. The reason for writing this paper is noted deviation from theoretical assumptions on the shape of reference surfaces in the various height systems. The deviation noted are in the part of relief characteristic over the deep karst and high mountains in one polygon of the II.NVT network, over the distance of 40 km.

Geoelectric surveys are among popular subsurface geophysical imaging techniques that provide significant insights into the electrical properties of subsurface targets. In this research, geoelectric modeling of travertine deposits at the Atashkohe region in Iran is performed through discretizing the physical model domain through structured (quadrilateral) and unstructured (triangular) meshes. This meshing tool captures the accurate borders of a rugged topography area along with any complex-shaped travertine sources. The modeling process was accomplished through utilizing an open-source python-based software called “ResIPy”, which handles all processing steps necessary for reliable forward and inversion of geoelectrical data (i.e., electrical resistivity and induced polarization). Three synthetic electrical models according to the geological background of the studied area were simulated to examine the efficiency of the 2D electrical survey in imaging the travertine building stone. Two types of structured and unstructured meshing were designed to cope with the rough topography surface and any intricate geometry of subsurface target. The inversion results of the synthetic models approved the accuracy and efficiency of both geoelectrical survey and data modeling in travertine exploration. Finally, as a case study, tomography data of electrical resistivity and chargeability taken from the Atashkohe travertine mine were geophysically modeled to infer geological sections along three 2D profiles surveyed in the region. Data were collected with an electrode spacing of 15 or 10 m through a combination of dipole-dipole and pole-dipole arrays. The results of inverse modeling indicated that the provided geological and electrical sections at depth could be related to travertine lenses and further survey can help delineation of these building rocks. Comparison of electrical models and stratigraphic column obtained from two drillings along one profile showed a relatively good agreement as well.

The Aegean is tectonically the most active and complex area in the Mediterranean region, including numerous active structures of all types of kinematics, often related to destructive seismic events. In such a tectonic environment, the estimation of rotational regime is decisive in the determination of the geodynamic and geotectonic evolution of this region. Satellite geodesy is a reliable way of estimating the rotation rate, while its comparison with palaeomagnetic data can lead to the identification of well-established patterns. To this direction, we use geodetic datasets recorded by permanent GPS/GNSS stations and we apply the triangulation methodology by combining three stations each time. Consequently, for each and every point of the 4733 extracted ones, the rotation rate and its pattern are calculated. Using the above results, two rotation models are proposed: one for the past 1 Myr and one for the past 5 Myr. In addition, the geostatistical analysis of the geodetically extracted results was carried out, in order to determine their spatial distribution. The comparison and combination of these two geodetically induced models with palaeomagnetic data, led to the delineation of distinct upper crust areas of uniform rotation within the Aegean region.

The structural styles and origin of the southern Atlas Front of Tunisia have long been controversial, and a detailed geophysical analysis was performed in order to aid in deciphering the Precambrian basement structural geometry. Wavelength filtering produced a residual gravity anomaly map which indicated gravity maxima over the Sidi Mansour and Chott El-Fejej basins with the maxima being caused by basement uplifts. Upward continuation, 3D-Euler deconvolution and 2.5D gravity forward modeling indicated that the depths of Mesozoic units ranged up 3 to 4 km with the thickest Mesozoic sediments being in the Metlaoui-Gafsa and Chott El-Jerid basins. 2.5D gravity modeling constrained by seismic reflection profiles and well data indicated that the tectonic configuration of the Precambrian basement is dominated by grabens, half-grabens and horsts with the Sidi Mansour and El-Fejej basins being located on horsts. Gravity modeling shows the thinning of the basement from south to north and from east to west. Earthquake focal mechanisms and hypocenters suggest that the deepest faults are located in the basement, beneath the Metlaoui and Sidi Mansour basins. Additionally, a number of hypocenters occur within the Mesozoic sediments. These hypocenters, together with the faults imaged by the seismic reflection profiles, indicate that the structures in the northern part of the study area have been controlled by a mixture of thin- and thick-skinned tectonics. The reactivation of the basement faults including the east-trending faults formed during the Alpine orogeny by the current compressive stress regime has led to the inversion of the horst and graben structures.

This study provides a comprehensive comparison of four different machine learning models including the group method of data handling (GMDH), M5 model tree (M5MT), multivariate adaptive regression spline (MARS), and Gaussian process regression (GPR) for predicting geoid undulation. For the first time, GMDH and M5MT were applied for this purpose. The obtained results were also compared with the classic inverse distance to a power (IDP) interpolation method. In order to assess the consistency of our results, two test sites with different topographic features were used for the evaluation of the models. In constructing the models, the geographic coordinate values and the geoid undulation value were used as inputs and output, respectively. Several statistical indices and rank analysis were used for evaluation of the models. According to the comparative results of all models in both test sites, the GMDH yielded the best performance among the developed models. The M5MT also exhibited acceptable results. Thus, it may be concluded that the proposed GMDH and M5MT have the potential to be alternative models that could assist geoscientists working with the geoid.

Inversion of magnetotelluric data is known as a nonlinear and ill-posed problem. To obtain meaningful and unique results, Tikhonov's regularization method is commonly used to solve it. The optimal selection of the regularization parameter is another important factor for achieving an ideal inverse modeling. The aim of the present study is to find the optimal value for the regularization parameter in a two-dimensional inversion of magnetotelluric data by introducing a novel method. Furthermore, the Lanczos bidiagonalization method has been used to speed up the inversion process. For this purpose, three common methods including L-Curve, Generalized Cross-Validation, and Discrepancy Principle were investigated and then compared with the Adaptive Regularization as a novel optimal method in the inversion of 2D magnetotelluric data. All methods were provided as the Matlab code by authors. A 2D synthetic MT data with 3% random noise and Bushli (Nir) geothermal field MT data in Ardabil province, in the NW of Iran, was used by the introduced method for demonstrating its efficiency. The obtained results affirm that despite the capability of all methods in selecting the regularization parameter, the introduced method is more efficient than other conventional methods in terms of required memory, elapsed time, convergence to the desired model in fewer iterations, and modeling accuracy. Morever, applying this method on real data demonstrates its ability to generate a realistic inverted model.